This invention relates generally to nonlinear optics. More particularly, it relates to generation of blue light for three-color laser light sources.
Color display systems often rely on three separate sources to produce three primary colors of light. The intensities of the three primary colors can then be varied and mixed to produce various different colors in a color image. The eye""s perception of color is related to the response of three different types of cells in the retina. Each type of cell responds to a different portion of the electromagnetic spectrum.
For the specific purpose of display or projection systems the best wavelength for xe2x80x9cbluexe2x80x9d light is about 450 nm (in vacuum). Such light is actually perceived by the human eye as a purplish-blue color as opposed to a pure blue. xe2x80x9cPurexe2x80x9d blue light is typically characterized by a wavelength in the range of about 460 nm to about 480 nm. The reason for using 450 nm can be explained using the chromaticity diagram of FIG. 1. Given three colors that can be located on the chromaticity diagram, it is only possible to create by addition colors which are on the interior of a triangle created by placing corner points at the three colors. It is clear from FIG. 1 that a wavelength of 450 nm is ideal. A display system based on a wavelength of 470 nm would create a situation where a number of well saturated purples and red-purples are outside the triangle and, thus, not accessible to the display system.
A single laser which has output at the three colors of red, green and blue would be valuable for projection displays. Development of such lasers has been hampered by difficulties in producing blue light at sufficient power levels for use in a display. One current approach to generating high power levels of blue light is to use Nd:YAG lasers operating at 1064 nm. The output of the laser is frequency doubled with a nonlinear crystal to 532 nm. The frequency-doubled output then pumps an OPO. One of the OPO output wavelengths is then summed with the 532-nm light to create the blue. Thus 2 nonlinear steps in 2 separate crystals are required to produce blue light from infrared laser light. Since each step requires crystals, and has limited efficiency, the overall system is expensive and inefficient. Furthermore, Nd:YAG lasers require water-cooling and resonator structures, which add to the complexity, bulk and cost of the system.
There is a need, therefore, for a compact, efficient and inexpensive blue lasers for red/green/blue displays.
Accordingly, it is a primary object of the present invention to provide a blue light source for projection displays. It is a further object of the invention to provide a blue laser that uses fewer and less-critically aligned components than previous systems. It is an additional object of the invention to provide a blue light source that uses only a single crystal.
These objects and advantages are attained by an apparatus and method for producing blue light and a fiber device having means for suppressing gain at an undesired wavelength. The apparatus generally comprises a light-generating fiber device optically coupled to an optical harmonic generator. The fiber device produces radiation at a power level sufficient to operate the optical harmonic generator. The optical harmonic generator increases a frequency of the radiation to produce a blue output radiation. The fiber device may be an oscillator or an amplifier, such as a Neodymium-doped cladding-pumped fiber amplifier. The fiber device may be pumped by a high intensity pump source to enhance the gain of radiation having a harmonic that is blue. For acceptable efficiency using the short-wavelength transition of Nd:glass fibers the pump power preferably remains above 50 Watts/mm2 along substantially the entire length of the fiber. The power of pumping radiation is preferably greater than about 100 Watts/mm2 and more preferably, about 500 Watts/mm2 or greater at a fiber entrance depending on the pumping configuration.
If the fiber device is a fiber amplifier, an oscillator may be optionally coupled to the fiber amplifier. The oscillator produces source radiation. The fiber amplifier amplifies the radiation produced by the oscillator. Suitable oscillators include mode locked lasers based on transitions in Nd:Glass, Nd:Vanadate, Nd:YLF, and other Nd materials and pulsed semiconductor lasers.
The fiber device typically produces infrared radiation having a frequency with a harmonic that falls in the blue portion of the visible spectrum. The optical harmonic generator generates blue light from the infrared light, by a non-linear harmonic generation process. The fiber device may include means to suppress gain of radiation having harmonics that are not blue. Such means include dopants, fiber gratings, and dichroic mirrors. In a specific embodiment, the gain suppression means suppresses gain at 1.05 xcexcm without suppressing gain at 0.91 xcexcm.
A first alternative means for optical gain suppression includes a fiber having a core surrounded by a cladding with a tunnel cladding disposed between the cladding and the core. Light of an undesired wavelength tunnels out of the core along the length of the fiber. The fiber thus has no bound modes at the undesired wavelength. A second alternative means for optical gain suppression includes a fiber that has been bent to a bend radius such that wavelength dependent losses caused by the bending attenuate radiation of the undesired wavelength.
The fiber device and blue laser apparatus find application as light sources for three-color light displays. Light sources based on the embodiments of the present invention are capable of producing blue light at an output power of order 1 watt or more.